The technology of the present disclosure relates to a fixing device and an image forming apparatus including the same.
An electrophotographic image forming apparatus includes a fixing device that fixes a toner image, which has been transferred to a sheet, to the sheet.
The fixing device includes a fixing roller having a heating device in a rotatable cylindrical part and a pressure roller brought into press-contact with the fixing roller to form a nip part. When a sheet (a paper, an OHP and the like) with a toner image formed thereon passes through between the fixing roller and the pressure roller, the toner image is heated and pressurized and is fixed to the sheet at that time.
As the heating device, there has been proposed a device including a central heater that mainly heats an axial center part of the fixing roller and an end heater that mainly heats an axial end part of the fixing roller. In the vicinity of the surface of the fixing roller, temperature detection sensors are fixedly disposed at the axial center part and end part, respectively. In the heating device, the heaters are controlled based on temperatures detected by the temperature detection sensors such that the temperature of both end parts of the fixing roller is higher than that of an intermediate part thereof. In this way, a reverse crown shape can be imparted to the sheet to prevent wrinkles from occurring.
As described above, in the heating device including the central heater and the end heater, heating ratio control is generally performed to change a heating ratio between the central heater and the end heater according to a width size of the sheet. In a non-passage area (that is, an area outside the width direction of a sheet passage area) of a sheet in the fixing roller, since heat is not lost to the sheet, excessive temperature rise is likely to occur. In this regard, it has been proposed to perform the heating ratio control in order to prevent the excessive temperature rise.
FIG. 16 is a graph illustrating the heat distribution of the central heater and the heat distribution of the end heater in the related fixing device in which the heating ratio control is performed. The central heater is set to have a large heat distribution (heat generation amount) at the center of the fixing roller and the end heater is set to have a large heat distribution (heat generation amount) at the end part of the fixing roller.
Furthermore, in the heating ratio control, for example, the lighting ratio of the end heater is lowered as the width size of the sheet is smaller. FIG. 17 is a graph illustrating the temperature distribution on the surface of the fixing roller when the heating ratio control is performed. It can be seen that the temperature of the non-passage area of the sheet on the surface of the fixing roller is suppressed to a threshold temperature or less when any of A4 to A6 size sheets is used (excessive temperature rise is prevented).